Recently micro-machined ultrasound transducers (MUT) have been developed. Micro-machined ultrasound transducers have been fabricated in two design approaches, one using a ceramic layer with piezoelectric properties (pMUT) and another using a membrane (or diaphragm) and substrate with electrodes (or electrode plates) forming a capacitor, a so-called capacitive micro-machined ultrasound transducer (cMUT).
A cMUT cell comprises a cavity underneath the membrane. For receiving ultrasound waves, ultrasound waves cause the membrane to move or vibrate and the variation in the capacitance between the electrodes can be detected. Thereby the ultrasound waves are transformed into a corresponding electrical signal. Conversely, an electrical signal applied to the electrodes causes the membrane to move or vibrate and thereby transmitting ultrasound waves. The mechanism by which this receiving or transmitting of waves occurs is called electromechanical coupling, or briefly, “coupling.”
Initially, cMUT cells were produced to operate in what is known as an “uncollapsed” mode. The conventional “uncollapsed” cMUT cell is essentially a non-linear device, where the coupling strongly depends on the bias voltage applied between the electrodes.
In order to solve this problem, so-called “pre-collapsed” cMUT cells have recently been developed. In a pre-collapsed cMUT cell a part or region of the membrane is collapsed to the bottom of the cavity (or substrate), the so-called “collapsed region”. In one type of “pre-collapsed” cMUT cells the membrane is permanently collapsed or fixed to the substrate, and in another type of “pre-collapsed” cMUT cells the membrane is only temporarily collapsed to the substrate (e.g. only during operation).
Over a wide range of bias voltages the coupling of a pre-collapsed cMUT cell is substantially bias voltage-independent, which makes the cMUT cell much more linear. Further, over the normal voltage range of use, such a “pre-collapsed” cMUT cell provides greater coupling than an “uncollapsed” cMUT cell. The reason is that the cavity or gap between the fixed bottom electrode of the substrate and the flexible top electrode of the membrane determines the capacitance of the cell and therefore its coupling. It is known that the highest coupling is obtained immediately adjacent to the point where the membrane is pulled into contact with the substrate. The region right around the collapsed region has the smallest gap, and therefore makes the greatest contribution to coupling.
A pre-collapsed cMUT cell is for example disclosed in WO 2010/097729 A1. WO 2010/097729 A1 discloses a cMUT transducer cell suitable for use in an ultrasonic cMUT transducer array having a membrane with a first electrode, a substrate with a second electrode, and a cavity between the membrane and the substrate. The cMUT is operated in a pre-collapsed state by biasing the membrane to a collapsed condition with the floor of the cavity, and a lens is cast over the collapsed membrane. When the lens material has polymerized or is of a sufficient stiffness, the bias voltage is removed and the lens material retains the membrane in the collapsed state.
However, there is still a need to provide a pre-collapsed capacitive micro-machined transducer cell with improved performance.